Remove VotableInterface from PonderQuestion

Summary: Ref T6920, This removes the PonderVotableInterface from PonderQuestion and assocaited code. Also... never used?

Test Plan: Visit Ponder, See List, New Question, Add Answer.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T6920

Differential Revision: https://secure.phabricator.com/D13833